Patent Application
20090111065
1. Field of the Invention
The present invention generally relates to machine controls and more particularly to the control of combustion in a burner for heating water or other substances by controlling air flow into the burner responsive to changes in physical parameters affecting air and or fuel density.
2. Background and Description of the Prior Art
Burners for machine systems such as water heater boilers for example, generally mix a fuel in gas or liquid form with air to provide a source of heat. Efficient combustion occurs when (a) the ratio of the mass of air to the mass of fuel is held within a small range of values centered on approximately 18-to-1, and (b) sufficient air is mixed with the fuel to ensure combustion of all of the fuel plus some small amount of “excess air.” Generally, sufficient air is provided when the amount of excess air is approximately 15%, which corresponds with an air-fuel ratio of approximately 18-to-1. If the excess air exceeds about 15%, some of the heat produced is consumed heating the excess air and is thus not available for heating the water in the boiler. Thus, it is important to maintain a stable and relatively low excess air level.
However, unless the burner is operated in an atmosphere of substantially constant air temperature and barometric pressure, the setting of operating controls for the burner is at best only a rough approximation to an optimum level for efficient combustion over normal variations in temperature. Thus, these settings require a substantial offset to compensate for changes in the air temperature. The result is that excess air values often exceed the 15% figure by a wide margin, to as much as 30% or more, when the combustion air temperature changes, placing an extra burden upon the heat energy produced upon the burner. Such a situation may occur, for example, when the temperature may vary as much as 20° F. to 30° F. or more over a 24 hour period, or as much as 80° F. to 100° F. through seasonal variations. To compensate for such variations, some burner efficiency, and some fuel consumption, is traded off for ensuring complete combustion at all times to minimize unburned fuel and emissions.
Most burners built today use a “Volume Control” system to control the flow of fuel and air. On gas fueled burners, the fuel pressure is controlled with a regulating valve, and the correct flow rate is obtained with an orifice. The orifice may be fixed for “On-Off” firing or it may be a control valve (like a butterfly valve) which can be opened and closed to allow more or less fuel in. The combustion air is controlled in a similar manner, using a fixed orifice for “On-Off” air flow control and an air damper for modulating air control.
Conventional volume control systems for water heater burners are subject to errors in the control of the air and fuel rate because the correct proportions of air and fuel are defined by the mass flow not volume flow. For each pound of natural gas provided to the burner, a corresponding quantity of air is required (about 18 pounds of air). According to the gas laws, the mass provided by a given volume of air can vary according to its temperature and the barometric pressure. Thus, the ratio of mass to volume is defined as the density of a gas, and can be defined mathematically for our purposes as,
Actual Density=(Std. density)×(absolute pressure/std pressure)×(std temperature/absolute temperature), {Eqn. 1}
where:
Density=weight of gas per unit volume of gas (lb/ft3 of gas at the stated pressure and temperature), and
Std. density=density of the gas at standard conditions (0.0765 lb/ft3 for air at 60° F. and 29.92″ Hg), where:
Absolute pressure=gauge pressure+barometric pressure of the current condition;
Std pressure=standard pressure, 29.92″ Hg (barometric pressure);
Std temperature=standard temperature, 60° F.; and
Absolute temperature=460+the temperature in ° F. of the gas.
These changes in density can result in large changes in the air-fuel ratio and the excess air of the burner combustion. For example, a difference of a combustion air temperature change from 120° F. on a hot afternoon to 40° F. on a cool morning will result in an increase in excess air of about 14%. This means that the burner is passing through 14% more excess air at 40° F. than at 120° F., and heating this air from 40° F. to the stack temperature (which is often around 500° F.) requires proportionately more fuel. This significantly reduces the efficiency of the boiler-burner package, making it more expensive to operate.
Oil fueled systems are not subject to the same density variations as a gas fuel system, because the liquid oil has a very small change in properties with temperature and pressure. For oil firing, the temperature generally must be controlled to maintain good atomization. Moreover, the oil pressures are so much higher than atmospheric pressure that the change in atmospheric (i.e., barometric) pressure has little effect. The concept of density change can be applied to oil flow, but it offers a much smaller improvement.
The impact of temperature and pressure variation is seen in the limitations and alternate control methods and systems used by burner manufacturers. Following are listed some typical methods that burner manufacturers use to solve these problems.
The concept of a “Fully Metered System” or “Full Metered Cross Limited Control System,” as described in (f) above, is not new. These systems have been used in the industry for many years. However, such systems are very complex and expensive, and only used in a very small number of special applications where the added performance justifies the cost and complexity.
Therefore, substantial industry-wide savings could be realized if a simple, low cost system or method were available that offers the control and efficiency of a fully metered system without the complexity and cost, and which is simple, reliable, and can be installed without major modifications to the burner and/or the structure of the water heater or other heating system. Such a system would provide a practical and economical alternative means of improving the efficiency of countless water heating and other types of heating systems in use.
Accordingly, an advance in the state of the art is disclosed that applies corrections to the mass flow rate of combustion air into a forced-draft burner for a water heater or other heating system, and thus the air-fuel ratio, by directly measuring the combustion air temperature and/or the barometric pressure of the combustion air, and using these measurements to develop a fan speed drive signal that corrects the volume of air inlet to the burner without the use of the complex and expensive fully metered control systems, or elaborate feedback systems, or systems that require real-time combustion analysis, and the like.
In one embodiment, an apparatus for controlling air flow into a burner responsive to parameter variations affecting air density is disclosed comprising: a fan motor for driving an air inlet fan of the oil fueled burner; a barometric pressure sensor for providing a first indicator signal to a controller; a combustion air temperature sensor for providing a second indicator signal to the controller; and a controller for receiving the first and second indicator signals at respective first and second inputs and processing them according to a predetermined relationship to provide a fan speed drive signal from a controller output coupled to the fan motor. In one aspect of this embodiment the controller includes a PLC and a variable frequency drive system. In another embodiment, the controller includes a PLC and a variable DC voltage drive system.
In another embodiment, a method of combustion control in a burner is disclosed comprising the step of processing both a first signal corresponding to an absolute barometric pressure measurement and a second signal corresponding to a combustion air temperature measurement in a controller to generate a variable frequency fan speed drive signal for coupling to an AC motor, or a variable amplitude fan speed drive signal for coupling to a DC motor, for driving an air inlet fan of the burner. In one aspect of this embodiment, the method regulates the fan speed responsive to changes in the first and second signals to vary the air flow volume into the burner, such that the fan speed varies inversely with changes in absolute barometric pressure and directly with changes in the combustion air temperature.
In another embodiment an apparatus for controlling air flow into a burner responsive to parameter variations affecting air density is disclosed comprising: a fan motor for driving an air inlet fan of the burner; a barometric pressure sensor for providing an electrical signal proportional to air density in the vicinity of the burner to a controller; and a controller for receiving the electrical signal at a control input thereof and processing it according to a predetermined relationship to provide a fan speed drive signal from a controller output to the fan motor.
In yet another embodiment an apparatus for controlling air flow into a burner responsive to parameter variations affecting air density is disclosed comprising: a fan motor for driving an air inlet fan of the burner; a combustion air temperature sensor for providing an electrical signal inversely proportional to air density in the vicinity of the burner to a controller; and a controller for receiving the electrical signal at a control input thereof and processing it according to a predetermined relationship to provide a fan speed drive signal from a controller output to the fan motor.
In still another embodiment an apparatus for controlling air flow into a burner for heating water responsive to parameter variations affecting air and fuel density is disclosed comprising: a fan motor for driving an air inlet fan of the burner; one or more sensing devices selected from the group consisting of a barometric pressure sensor for providing a first indicator signal to a controller, a combustion air temperature sensor for providing a second indicator signal to the controller, a fuel temperature sensor for providing a third indicator signal to the controller, and a fuel pressure sensor for providing a fourth indicator signal to the controller; and a controller for receiving one or more of the first, second, third, and fourth indicator signals at respective inputs thereto and processing them according to a predetermined relationship to provide a fan speed drive signal from a controller output to the fan motor.
The embodiment of the present invention described herein is not intended to be limiting but to illustrate the principles and the application of the invention. The present embodiment applies corrections for both combustion air temperature and barometric pressure to an illustrative water heater burner system. As used in the following description, combustion air is the air inlet to the burner, whether it is the ambient air at the inlet to the burner, indoor air ducted to the burner air inlet, or outside air ducted to the burner air inlet. However, the invention may be adapted to use the correction systems individually for temperature or pressure or to either gas-fueled or oil-fueled burners, depending upon the particular application. Further, while the embodiment to be described focuses on the particular control mechanisms that may be embodied in an illustrative water heater system, the present invention is readily adaptable to burners used in other applications such as steam boilers, kilns, foundries, etc. Moreover, because the present invention provides a control mechanism that operates independently of the usual mechanisms found in the illustrative water heating systems that utilize burners, many of the structural and operating details of these usual mechanisms of the water heaters, well known to persons skilled in the art but unrelated to the present invention, are not described herein.
Regulating the operation of a burner involves the application of several well-known relationships for gases. The density of a gas D is determined by the amount of the gas per unit volume, or, mass/vol or, D=m/V. The Ideal Gas Law states that the volume of a gas is related to the temperature and pressure by the formula P×V=k×T, where P=pressure; V=volume, T=temperature, and k=constant. Restated, this relationship is V=(k×T)÷P, or, simply V ∝T/P. Thus simplified, the density D∝m÷(T/P), or, D∝m(P/T). In words, density is proportional to pressure and inversely proportional to temperature. In a burner, to maintain an efficient combustion ratio, the parameter of interest is the mass flow rate of the air or the gas into the burner. Since the mass of a gas varies with its density, the mass flow rate of the gas (or air) varies with barometric pressure and inversely with ambient temperature.
The present invention described herein takes advantage of the dependence of the density of air used in a combustion mixture with a gas or oil (liquid) fuel upon the combustion air temperature and barometric (atmospheric) pressure of the air inlet to a burner for an illustrative water heater. This relationship, since it defines the effect of combustion air temperature and barometric pressure upon the mass of air and thereby the mass flow of air inlet to the burner, enables control of the air-fuel ratio, the ratio of the masses of the air and fuel, based on the outputs of combustion air temperature and barometric pressure sensors placed in the inlet side of the burner system. To say it another way, the system applies corrections to the air flow in response to variations in those attributes that would alter the mass flow rate and upset the air-fuel ratio of the mixture into the burner. The control provides correction of the air-fuel ratio for the changes in combustion air temperature and pressure that may occur during normal operation of the burner, whether the variations take place daily or seasonally. Not only is the air-fuel ratio held within more efficient limits, but the excess air is also controlled more closely to the preferred range of air-fuel ratios, providing a burner system that will have fewer maintenance problems caused by flame instability when operating at very high air-fuel ratios. The result is more reliability and a savings of fuel and energy costs provided by a more efficient burner. Moreover, because the control reduces the fan speed, it will also provide a savings of electrical energy, an inherent benefit of using a variable frequency drive (“VFD”) for use with AC fan motors, or a variable speed drive (“VSD”) for use with DC fan motors, that is described herein.
One important operating parameter of burners that is related to the air-fuel ratio for efficient combustion and to the stability of the combustion that occurs in the burner is called “excess air.” The optimum air-fuel ratio of the masses of air and fuel flowing into the burner for efficient combustion is approximately 16 pounds of air for every pound of fuel consumed, i.e., 16 to 1. If less air is inlet to the burner for each pound of fuel, the result is lower heat output and the emission of unburned fuel, representing wasteful operation. If more than 16 pounds of air is inlet to the burner for each pound of fuel, some of the energy in the fuel is used to heat the excess air and the combustion is operating too lean, representing inefficient operation. It turns out that some small amount of excess air—e.g., 10% to 15%—is preferred to ensure complete burning of the fuel, resulting in an air-fuel ratio of approximately 18 pounds of air to one pound of fuel. Thus, a measure of the combustion efficiency is the amount of excess air that is permitted. Normally, a range of percentages, from about 10% to 30% is allowed, which accommodates a range of operating conditions such as air temperature and other parameters that affect the density of the air inlet for combustion, and ultimately, the air to fuel ratio.
One condition that can occur if the excess air becomes too large a percentage of the optimum mass flow rate of the air is called “flame instability.” This occurs when there is insufficient fuel involved in the combustion process, i.e., an overly lean mixture of fuel in proportion to the available air. The resulting flame is starved for fuel, making it uneven and unstable. An unstable flame may cause the burner to “huff and puff,” as it tries to adjust to the excessive amount of air, with very poor efficiency and low or intermittent heat output. In severe cases, the burner may shake with the uneven burning, possibly leading to vibration and damage to burner structure, etc.
The present invention, by fine tuning the air to fuel ratio in response to factors that affect the density of the air and, to a lesser extent, the fuel in some applications, acts to prevent instability and to maintain the excess air within a smaller range that is closer to the optimum value over a wider range of temperatures and pressures. Thus, maintaining the excess air within a narrower range results in direct energy savings and improved efficiency. The present invention, as will be apparent from the following description, is also simple, easy to adapt to existing systems, and is relatively low in cost. It also results in a smoother operating burner system and improved longevity.
The system and method of the present invention may be retrofitted to existing burners without modification to the burner components. Since the system and method involves control—i.e., electrical changes—only of the inlet air fan, it is independent of the burner hardware and thus does not involve or affect the burner itself, which operates according to its own control loop. Moreover, it is low in cost, requiring only the addition of a temperature and/or a barometric pressure sensing devices, an interface circuit or system such as a VFD system or a VSD system (also called VFDS or VSDS, respectively herein), all of which are nominal cost items, to implement the system.
The interface circuit or system receives the signals from the sensing devices and processes them according to a well-defined transfer function, producing a fan speed drive signal that varies the speed of the AC motor driving the inlet air, aka the “combustion air” fan. The fan speed drive signal may be a variable amplitude DC voltage or a variable frequency AC voltage, depending upon the type of motor used in the system. The present invention quantifies, as a percentage of flow, the change in air density caused by the changes in combustion air temperature and barometric pressure, as defined by the Ideal Gas Law. The Fan Laws state that, at a constant fan speed, the air volume provided for the combustion of the fuel will remain the same even though the density has changed, resulting in a mass flow change directly related to the density change caused by changes in combustion air temperature and barometric pressure. Further, the Fan Laws state that a change in fan speed will result in a proportional volume change. Thus, changing the fan speed the same percentage as the resulting density changes will correct the density change and provide a constant mass flow of air for combustion. For example, if the density relations indicate that the mass flow rate is reduced 3% because of an increase in temperature, the system can increase the fan speed by 3% to correct for the change in density caused by the change in temperature.
In practice, persons skilled in the art will recognize that, while the Ideal Gas Law and the Fan Laws provide the foundation of the control strategy embodied in the present invention, some minor variations in the actual flow characteristics may be noticed in real world applications. In such cases, engineering design and experimentation are relied upon to make needed adjustments or to compensate for these variations from the ideal case. The control described herein, because it is configured to affect only the fan speed, is readily adaptable to existing systems largely without affecting the control mechanisms already in place. Such mechanisms include linkage or parallel positioning systems that control the operation of valves through mechanical linkages, from those that provide a simple ON-OFF, LOW-HIGH-LOW control to those operated by multiple linkages connected to a single actuator or to those providing continuously variable control operated by a modulation motor. Actuators and modulators may be controlled by switches or electronics.
Referring to
Continuing with
Returning to the illustrated embodiment, the VFD is also coupled to the control panel 68 via the line 70 to enable it to be responsive to other control parameters and conditions. Line 70 is typically a cable containing numerous connections to the control panel 68. The control panel 68 controls the operations of the VFD 64 in response to a variety of conditions to provide efficient operation, save energy, and maximize the safety and reliability of the burner. The AC motor 38 may be closely controlled in start/stop, speed control, ramping up/down of the fan 36. Operating limits are also closely controlled to avoid damage or unsafe conditions. While important to the operation of the water heater and burner system, these functions of the control panel 68 are not relevant to the present invention and will not be described further herein. Thus the present invention may be implemented or retrofitted to existing equipment at nominal cost and without requiring modifications to the system other than adding several nominal cost components and changing some of the wiring.
Two sensors are provided in the controller 16 for the burner system 14 shown in
The pressure and temperature sensor outputs are coupled respectively via lines 60 and 62 to a circuit or circuit system such as a PLC 58, to be processed and converted to a fan speed signal under program control. Persons skilled in the art will realize that a specially-designed circuit could be used for the circuit system at block 58. However, a programmable logic controller (PLC) is convenient because it is an off-the-shelf component that can receive multiple inputs and can be programmed for multiple outputs. Further, through its ability to respond to programmed instructions, it can apply an appropriate transfer function to the processing of the input indicator signals to produce the fan speed signal at the output of the PLC via the line 66 coupled to the VFD 64. In the illustrative example, a suitable PLC device is a Part No. HE-XE105 manufactured by Horner APG, LLC, Indianapolis, Ind. 46201, and www.heapg.com. The output of the PLC 58 may be coupled to an input of a VFD 64. The VFD 64 is a machine control to be described that is present in the AC supply circuit to the fan motor 38. In the present invention, the VFD 64 is utilized to also respond to the fan sped signal as a control input from the PLC 58 by varying the frequency of the AC voltage to change the speed of the fan motor 38. In other embodiments having only a single control input, such as either temperature or barometric pressure, that control input (sensor output) can be connected directly to the VFD 64 as long as the signal complies with the standard 4 mA to 20 mA range.
The VFD 64 is a standard off-the-shelf component that provides a control method for correcting the air-fuel combustion ratio for changes in the ambient temperature and barometric pressure. As noted herein above, the flow rate of the air 34 inlet to the firing head 30 is a direct, linear function of the speed of the fan 36 because of the fan law. The VFD 64 in this example z operates from a three phase AC voltage supply via the lines 72 and includes a rectifier, a frequency inverter, and a control section as internal circuitry (not shown) to regulate the frequency of the output waveforms in accordance with the fan speed signal from the PLC 58. The fan speed signal input to the VFD 64 from the PLC 58 may be a DC current, such as a 4 mA to 20 mA current, or it may be a DC voltage varying in the range of 0 to 10 Volts DC, for example, according to industry standard practice.
The VFD 64 generates a variable frequency AC voltage to drive the AC operated fan motor 38. The fan motor 38, which nominally operates at 3500 RPM (in this example) when the AC supply voltage is 60 Hz, may be slowed down by reducing the frequency of the AC voltage generated by the VFD 64. This variation in the AC voltage output frequency is proportional to the fan speed drive signal supplied by the PLC 58 and coupled to an input of the VFD via the line 66. The VFD is a device known in the industry as a general machinery drive. In the illustrated embodiment, the VFD may be a type ACS350 manufactured by ABB Inc., New Berlin, Wis. 53151, and www.abb.us/drives.
In an alternative embodiment that is not illustrated herein but will readily occur to persons skilled in the art, the VFD 64 may be replaced by a variable speed drive (“VSD”) that provides a direct current fan speed drive voltage for controlling a DC operated fan motor. Substitution of a DC motor for an AC motor does not change the present invention, is contemplated as falling within the scope of the present invention, and is merely a functionally equivalent choice made to satisfy a particular application. Some burners for heating water, or used in other systems may utilize a DC motor as efficiently as an AC motor. In such applications, a variable speed drive or VSD is substituted for the VFD. A VSD may be configured to be responsive to a DC fan speed signal output to the VSD by the PLC.
While the present invention is illustrated herein by an embodiment having control of both the combustion air temperature and the barometric pressure, other applications may use differing embodiments, considering factors such as the following. For example, in gas burners, both the air and gas supply pressures are referenced to the barometric pressure. The inlet pressure to the fan is the atmospheric pressure, and the gas pressure regulator controls to some pressure over the atmospheric pressure. Thus, in the case of a gas burner, these two pressure effects change in the same direction, and in most cases a correction to the mass flow of the air inlet is required only for variations in the ambient temperature. However, in gas burners with a vented gas pressure regulator, a slightly modified correlation may be required because the barometric pressure change will also change the gas pressure. The correction adjustment may be made in the PLC 58 by referencing the regulated gas pressure. In the case of an oil burner, since the variations in atmospheric pressure will affect the air mass flow while the oil mass flow rate remains unchanged, a correction to the mass flow of the air inlet is required for variations in both the combustion air temperature and the atmospheric (i.e., barometric) pressure.
Referring to
The VFD 64 is a machine control unit connected between the three phase AC supply source and the AC supply terminals of the fan motor 38. Thus, the L1 line in cable 72 connects to terminal U1 of the VFD 64 and terminal U2 of the VFD 64 connects to an L1 terminal of the fan motor 38. Similarly, line L2 from the source connects via cable 72 through terminals V1, V2 to an L2 terminal of the fan motor 38 and an L3 line in cable 72 connects through terminals W1, W2 to an L3 terminal of the fan motor 38. A ground connection from terminal PE of the VFD 64 is provided on the AC source side and a ground connection from the terminal PE on the output of the VFD 64 is provided to the frame of the fan motor 38. The cable 44 from the VFD 64 may be shielded, with the shield connected to the PE terminal of the VFD 64. The control panel 68 shown in
The linear speed control characteristic provided by the VFD 64 enables a simple relationship between the variations in the sensed parameters and the speed of the fan motor 38 to be established by the control section 16. For example, in a typical application where the air temperature is expected to vary between 50° F. and 120° F., the maximum motor speed, 3500 rpm at 60 Hz, may be set to correspond to the maximum temperature, 120° F. (where the air has the lowest density) and the minimum motor speed may be set to, for example, 3077 rpm at the 50° F. temperature of the ambient air where the air has the highest density. The speed of the fan motor 38 is held constant above 120° F. and below 50° F., and varies linearly between these two temperatures. These limits are typically determined by factory settings. The factory settings cover all the expected temperatures of operation, the fuel input rate and the amount of air required to completely and efficiently burn all of the fuel, and standard temperature and barometric pressure for the region where the system will be operating. An example of the calculation to determine the speed of the fan motor 38 at 50° F. follows.
Consider the application where the air temperature varies from 120° F. (condition 1) to 50° F. (condition 2), and the normal barometric pressure is 28.7″ Hg. We will use several standard values and relations in the following calculations. They are:
Density=weight of gas per volume of gas (lb/ft3 of gas at the stated pressure and temperature);
Std. density=density of the gas at standard conditions (0.0765 lb/ft3 for air at 60° F. and 29.92″ Hg);
Absolute pressure=gauge pressure+barometric pressure of the current condition;
Std pressure=standard pressure, 29.92″ Hg (barometric pressure);
Std temperature=standard temperature, 60° F.; and
Absolute temperature=460+° F. of the gas.
Based on the known fuel input, the burner requires 10,000 pounds per hour of air to completely and efficiently burner all of the fuel provided by the burner. The following analysis would be used to generate the control strategy.
The densities of the air at the two conditions are (from Eqn. 1);
Density 1=0.0765×(28.7/29.92)×(460+60)/(460+120)=0.06579 lb/cuft
Density 2=0.0765×(28.7/29.92)×(460+60)/(460+50)=0.07482 lb/cuft
The required fan output for each condition will be, using
Fan Actual Cubic Feet per Minute (ACFM)=(lb air/hr)/(density×60 min/hr) {Eqn. 2)
ACFM1=10,000/(0.06579×60)=2533 CFM
ACFM2=10,000/(0.07482×60)=2228 CFM
Where the values are;
Lb air/hr=pounds of air required per hour (as stated in this example);
Standard air density=0.0765 lb/ft3;
Standard air pressure=29.92″ Hg;
Local air pressure=28.7″ Hg;
Air temperature at condition 1=120° F.;
Air temperature at condition 2=50° F.; and
RPM=revolutions per minute.
The burner was setup under condition #1 at 120° F., which is the lowest air density. The combustion air motor and fan are operating at 3500 RPM and the air damper is adjusted to generate a flow of 2533 CFM, which provided enough air to completely burn the fuel and some minimal amount of excess air, for good combustion efficiency.
At condition #2, the fan will generate the same volume of air (based on fan laws), and since the density is much higher (more pounds of air per volume at this lower air temperature) the burner would normally have much more air then needed for combustion. A higher excess air rate would result in lower combustion efficiency. The system of the present invention will change the fan speed to match the changes in air temperature, and provide the same mass of air to the burner firing head 30. The new fan speed required to obtain a volume flow of 2228 CFM is,
Where,
The foregoing example illustrates an application of the present invention to a water heater burner system wherein the combustion air temperature alone is used as a control parameter to vary the speed of the fan motor 38. This example is simple and low cost, making it especially adaptable to smaller burners with lower fuel costs and lower payback opportunity. In this application, the PLC is not needed because the 4 to 20 mA analog control input to the VFD 64 is available. The VFD device generally has this capability through its built-in single loop controller to convert the DC control input to the fan speed control signal. This particular embodiment thus does not require any programming and would be transparent to the start-up technician and in use. Persons skilled in the art will readily be able to adapt the invention to their specific system based on the description provided in the foregoing example.
Other applications of the present invention include a simple pressure control package for burners that again utilizes the single loop controller of the VFD 64 and a barometric sensor such as the sensor 50 and probe 52 combination described herein above. The process for configuring the system is similar, based on initial conditions defined for two different air densities and the corresponding fan outputs (ACFM1 and ACFM2) calculated from: (amount of air required, in lb., for the given amount of fuel)÷(air density, in lb./cu. ft.) for each of the two conditions. For a hypothetical atmospheric pressure range of 27.7 in. (condition 1) to 29.7 in. (condition 2), a temperature of 85° F. and 10,000 lb. of air required to burn the fuel, ACFM1=2466 CFM and ACFM2=2300 CFM. At condition 1, the RPM, is set to 3500 RPM for apressure of 29.7 in. Then RPM2 is determined by: RPM2=3500 (2300÷2466)=3264 RPM. Notice in this example that the highest fan speed is set to the lower pressure boundary, where the density of the air is lower. As the pressure rises, the density of the air increases, and the fan speed necessary to maintain the correct CFM must be reduced.
In another application of the present invention for water heaters, both combustion air temperature and barometric pressure corrections can be implemented. The system is much like the illustrated embodiment described herein above. From the previous examples of single control elements, the correction for air temperature and pressure has been defined. They can be combined in the following manner, wherein the calculations are performed in the PLC responsive to inputs from both types of sensors. Correction factors for the ambient air temperature and the barometric pressure are defined as follows:
K
T=(460+Tair)/(460+Tmax); and
K
P
=Bplow/BPair.
Thus, the fan speed is determined by:
Speed=3500 RPM×KT×KP,
KT=Temperature correction factor (dimensionless);
KP=Barometric pressure correction factor (dimensionless);
BPair=current barometric pressure, Hg, in.;
BPlow=lowest barometric pressure, Hg, in.;
Tair=current air temperature, ° F.;
Tairmax=the highest expected combustion air temperature ° F.; and
Speed=controlled RPM of the combustion air fan motor.
These calculations provide a set of relationships—which may be represented by a family of characteristic curves, if plotted (i.e., one curve for each increment of barometric pressure, when the axes are motor speed vs. combustion air)—where the different barometric pressures would be identified with multiple lines. These operations would be performed on a continuous manner, where the fan speed drive signal is always calculated and delivered to the VFD, and the fan always operates at the correct speed for the operating conditions. When the unit is initially setup, it will be calibrated to the correct mass flow, as measured by a combustion analysis performed at startup.
The foregoing are just a few of the examples of combustion control through applying measurements of temperature and pressure of the ingredients of the combustion process. Other potential applications include controls based on: gas fuel temperature; combined fuel temperature, combustion air temperature and barometric pressure; and outside ducted combustion air temperature. Any combination of combustion air temperatures, barometric pressure, gas fuel temperature and gas fuel pressure can be used by applying the Ideal Gas Law and the Fan Laws.
The present invention may even be used to correct the fan speed in a burner system that already uses a variable speed control to maintain a constant pressure at the air inlet of the burner, between the air damper and the fan. In such a variable motor speed control system, a pressure sensor is located between the air damper and fan inlet to measure the pressure at that location. A single loop controller reads this pressure and is programmed to maintain a constant pressure, typically around −2.0″ w.c. (inches of water columr). Note, for reference, 27.7″ w.c. in a tube=1.0 pounds per square inch (“psi”). As the air damper opens, the pressure drops, and the control will increase the fan motor speed to maintain the set pressure. As the air damper opens, increasing the air supply to the burner, the firing rate is allowed to increase. If the air damper is located on the outlet side of the fan, the pressure will be positive instead of negative. This system has been used in many applications over the years. Typically, the motor will vary from about 1000 RPM at low fire up to 3500 RPM at high fire. The electrical use at the lower firing rates is considerably lower than the standard burner, and results in a significant electrical savings. Rebates from electric companies may be available for these applications.
In some applications, known as so-called “true variable speed systems,” where the fan speed is controlled over a large speed range, e.g., 1000 RPM to 3500 RPM, control based on temperature offers true savings. This is also true for combined sensing, such as temperature and pressure, yielding improved efficiency and savings. The present invention is primarily directed to and contemplated for use with systems in which substantial gains in efficiency can be realized by varying the fan motor speed over a narrower range, such as 2800 to 3500 RPM. Nevertheless, the principles of the present invention may readily be applied to control of the wider range of speeds, with corresponding improvements in efficiency and reduced operating costs.
To combine the electrical savings of the standard variable speed motor control with, for example, the air temperature control of the illustrated embodiment described herein above, the application of the air temperature adjustment would be accomplished using a “square law” that says the ratio of pressures equals the ratio of the flows squared, or
P
2
=P
1×(ACFM2÷ACFM1)2 {Eqn. 5}
Where,
P2=New pressure set point between the air damper and fan;
P1=Original pressure set point between the air damper and fan, −2.0″ wc;
ACFM1=air flow rate before temperature change; and
ACFM2=air flow rate required after temperature changes.
The ratio of old to new air flow is represents the volume air flow rate change required to maintain the same mass flow rate of the burner, which can be determined directly from the temperature change as done in the described embodiment, with the final form of:
P
2
=P
1×(460+Tair)/(460+Tairmax) {Eqn. 6}
Where,
Tair=current air temperature, ° F.;
Tairmax=the highest expected combustion air temperature ° F.;
Maximum air temperature=maximum expected air temperature ° F.; and
Absolute temperature of air=(460+air temperature ° F.).
A PLC is required to combine the readings of the pressure sensor and offset according the above (equation 6). This would be converted to a 4-20 mA signal that can be used by the single loop controller in the VFD, which will vary the combustion air motor speed to maintain the desired set point pressure.
While the invention is described in only several of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof. In the illustrative example, the control system is an electrical or electronic device, which is a typical implementation of machine control systems. In some electrically-based systems, substitutions may be made. For example, the PLC and/or the VFD or VSD may be replaced by a circuit specifically designed to process the sensor outputs and generate the particular kind of control or “fan speed signal.” Further, other systems may be more amenable to control systems based on hydraulic or pneumatic circuits for sensing operating parameters and generating corresponding outputs to maintain the mass flow rate of air inlet to a burner within an optimum range for high efficiency. In other systems, the control outputs may be derived from sensors that detect variations in fuel parameters and adjust the inlet air flow to maintain a predetermined combustion efficiency and performance.
